We study the mechanism of propagation of the L-A dsRNA virus of the yeast Saccharomyces cerevisiae, the host genes which it uses for this purpose and the mechanisms by which the host limits viral propagation to prevent viral cytopathology. We find that the N-terminal 1/4 of the L-A pol gene is needed for encapsidation of viral RNA, but that only gag is needed to assemble viral particles. We have found that pol encodes three ssRNA binding regions, one of which is cryptic, its activity being inhibited by a region C-terminal to this binding region. One of the ssRNA binding regions of pol is within the region necessary for packaging, suggesting that this is its in vivo role. The others are likely to be involved in RNA replication and transcription. The assembly process requires the MAK3 N- acetyltransferase that acetylates the N-terminus of gag. Stable particle formation also requires the MAK10 protein, particularly if the particles are completely full. The six SK1 genes comprise a host system to limit the replication of three unrelated dsRNA and ssRNA replicons. We have obtained evidence that they do this by limiting the translation of uncapped messages. We have isolated mutations in several chromosomal genes that result in substantial increases in the efficiency of the -1 ribosomal frameshifting that L-A uses to synthesize its gag-pol fusion protein. We find that the efficiency of frameshifting is critical to viral propagation, suggesting that drugs affecting this process may be useful against retroviruses. The [URE3] non-Mendelian genetic element of yeast requires for its propagation the URE2 chromosomal gene. We have evidence that [URE3] is a prion of yeast.